The UN will investigate an alleged chemical weapons attack in Syria after Britain spearheaded pressure in New York for a full inquiry into poison gas attacks in Syria's civil war.

Ban Ki-moon, the UN Secretary General, said Syrian regime allegations that rebel fighters had fired a chemical weapon in an attack near Aleppo that killed at least 25 on Tuesday would be investigated.

"My announcement should serve as an unequivocal reminder that the use of chemical weapons is a crime against humanity," the secretary-general said. "The international community needs full assurance that chemical weapons stockpiles are verifiably safeguarded."

Ban Ki-moon announces that the UN will investigate the use of chemical weapons in Syria

Philip Parham, Britain’s deputy UN ambassador said that the “facts need to be clarified,” adding that there was more than one report of chemicals being used in attacks.

“I emphasise reports plural. If chemical weapons have been used, this would be abhorrent, it would be very grave, it would warrant a serious response by the international community.”

However the inquiry’s scope was not extended to other instances involving the use of weapons of mass destruction.

Foreign Office officials take three separate incidents in which victims reported poison gas effects as credible in the two-year war.

Russia blocked attempts to widen the remit of the probe beyond the attack on the village of Khan al-Assad, near Aleppo.

Vitaly Churkin, Russia’s ambassador, said the Western initiative was an attempt “to delay” any investigation into claims of weapons use in Aleppo.

“Instead of launching those propaganda balloons I think that its much better to get our focus right and I hope this is what the secretary general is doing,” Mr Churkin said.

The opposition has accused the government of staging the Aleppo attack and another with chemical weapons at Atayba near Damascus.

US intelligence officials on Thursday said they had strong indications chemical weapons were not used in the attack. While officials will not entirely rule out the possibility, an official said that additional intelligence-gathering in recent days has led the US to believe more strongly that it was not a weaponised attack.

Two senior US senators meanwhile made a fresh demand for President Barack Obama to intervene in order to "ease the suffering of the Syrian people and protect US national security interests”.

Carl Levin, a Michigan Democrat who chairs the Senate armed services committee, and John McCain, the Arizona Republican and former presidential candidate, made their plea in a detailed open letter.

"We believe there are credible options at your disposal, including limited military options, that would require neither putting US troops on the ground nor acting unilaterally," the pair told Mr Obama.

They called for the establishment by the US and Nato allies of a "safe zone" in northern Syria protected by missile batteries that have already been deployed to southern Turkey.

Mr Levin and Mr McCain also urged the president to consider destroying regime aircraft and missile sites "using precision airstrikes" that would not require "pilots to fly into the reach of Syria’s air defences".

Finally they urged the White House to supply vetted rebel groups with "tactical intelligence and increased deliveries of food and medicine, fuel, communications equipment" as well as medical aid.

Hours after the inquiry was announced, a rebel suicide bomber penetrated the regimes ring of steel in central Damascus to kill the most senior Sunni Muslim mufti, Sheikh Muhammad al-Bouti.

Sheikh Bouti, who was killed along with dozens others in the Iman Mosque in the city centre gave trenchant support to the government in his weekly addresses at Friday prayers broadcast live on state television.

He had called the rebels “scum” in one widely circulated video on the internet.

After news of the attack broke, one state television station interrupted its regular programme to broadcast verses from the Koran, the Muslim holy book.

With the prospect of Syria’s large arsenal of weapons, including nerve agents, playing a role in Syria, William Hague is to push to amend the EU arms embargo today and tomorrow.

The Foreign Secretary travels to Dublin to press for changes that would allow arms shipments to the rebel fighters before the blanket ban is rolled over for three months at the end of May.

At a meeting three weeks ago, France and Britain pressed the EU to ease the arms embargo to allow the supply of non-lethal equipment to the rebels, Britain immediately pledging armoured vehicles and protective clothing.

Officials fear there will be no decision in the talks and warn that if the embargo is rolled over at the end of May it would tie the hands of the opposition’s Western backers until the end of September.